The court in Part III of its opinion indicates that in this case “no court has thus far considered how” to distinguish between official and unofficial acts.
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Moreover, Roberts continues, “the lower courts rendered their decisions on a highly expedited basis” and “did not analyze the conduct alleged in the indictment to decide which of it should be categorized as official and which unofficial” — and it wasn’t briefed before the Supreme Court.
Sotomoyer has a long dissent where she cherry-picks everything Trump supposedly did as part of his ‘conspiracy’ and then notes that the Constitution is silent about prosecuting _former_ Presidents for crimes... notwithstanding that Trump was actually the President at the time, of course, and she seems to walk a razor-thin line about preferring prosecution for official acts after leaving office.
At first glance, it seems as if she’s arguing this: suppose a (real) perp strikes an immunity plea deal with a D.A. to rat out co-conspirators for a crime. Everybody signs off and the guy is released. A month later, he’s re-arrested and prosecuted when the same D.A. says “oh, that immunity deal has expired now that you’re no longer in a position to help us.”